GTK: Check the Connection ID is positive non-zero before removing. The connection ID will be zero under two conditions: 1) The GTK signal handler failed to be created or 2) The Dynamic Windows handler uses an existing GTK handler Therefore we don't want to ASSERT when removing a DW handler when it uses an existing GTK handler. So check and warn if the CID is invalid at creation time, and skip invalid CIDs at removal time. Thanks Mark for pointing out this misbehavior.